Gypsum is a naturally occurring mineral typically found in old salt-lake beds, volcanic deposits and clay beds in the form of calcium sulfate dihydrate (CaSO.sub.4 .multidot.2H.sub.2 O). Gypsum is also formed as a by-product of various industrial processes such as, for example, the manufacture of titanium dioxide and the desulfurization of flue gases. Calcining, that is, heating the mineral to about 120.degree. C. drives off 11/2 molecules of water, leaving behind calcium sulfate hemihydrate (CaSO.sub.4 .multidot.1/2H.sub.2 O). Anhydrous calcium sulfate (CaSO.sub.4) can be produced by heating to a temperature of about 163.degree. C. Either of these forms of gypsum, referred to herein as "calcined gypsum", is capable of reacting with water to form calcium sulphate dihydrate, which is referred to herein as "set calcined gypsum."
Fire resistant gypsum board, commonly used as wall and ceiling partitions, consists of a stiff or rigid core of set calcined gypsum, usually about 1/2 or about 5/8 inch thick, sandwiched between two paper liners which form the outer surfaces of the board. Speaking generally, the core of fire resistant gypsum board is made from an aqueous slurry of calcined gypsum which has set to form a hard, fairly rigid product. Gypsum also is used in one or more layers of fire resistant barrier systems for roof coverings. One such system uses gypsum board in combination with glass fiber mats.
Fire resistant gypsum board must have certain basic properties in order to meet accepted standards which dictate whether the board is suitable for commercial use. When exposed to intense heat, such as that generated by fire in a burning building, the board, which is extensively used for constructing walls and ceilings, is expected to stay in place for a defined length of time where it functions to deter the spread of fire. Such properties are evaluated in fire resistant assembly tests which have been adopted by the industry. When subjected to standard fire resistant tests which simulate conditions in a burning building, it is not unusual for commercially available fire resistant board to remain in place for one to two hours during which time it is exposed to temperatures as high as 1850.degree. F.
Non-woven fiber mats may also be utilized as fire resistant backing sheets for materials such as doors, carpets or vinyl fabrics. The relative degree of fire resistance provided by fiber mats depends upon numerous factors, including the size and composition of the fibers, and the composition of the binder. However, these factors also contribute to the functional properties of the mat. Thus, the prior art in considering the development of a fire resistant mat considered the functional properties thereof in addition to the degree of fire resistance desired.
Moreover, compliance with various fire resistance testing methods is an important consideration in developing fire resistant mats or barrier systems because many regulatory agencies and building codes rely on these tests in determining the acceptance of building materials used in various applications. Insurance rates may also be affected by compliance with fire resistance testing methods. One of these widely accepted classifications for fire resistance is the Underwriters Laboratories Inc. (UL) 790 Burning Brand Test. This test classifies flame resistance from Class A through Class C. Materials obtaining a Class A rating are the most fire resistant.
The Class A UL-790 Burning Brand Test simulates a burning tree branch falling onto a roof system in a twelve mile per hour wind. The "Brand" is a 12.times.12.times.2.25 inch wood composite that is pre-ignited and placed on a test roof midsection at a prescribed location relative to a seam joining three plywood panels. The Class B test is similar, but utilizes two brands placed at two locations on the roof section considered most vulnerable. The Class B brands are each 6.times.6.times.2.25 inch wood composite. The test is passed if the roof does not burn through and produce a self-sustained flame within 90 minutes. During the test, the roof system experiences temperatures approaching 2000.degree. F.
Fire resistant barrier systems capable of meeting Class A UL-790 requirements are typically heavy, bulky, inflexible, difficult to use and costly. In a non-woven fiber mat or barrier system that is used as a fire resistant backing, it is desirable to obtain the optimum level of fire resistance without sacrificing the functional properties of the mat or the various components of the barrier system and at the same time reducing its weight and cost.
There remains a need for a light-weight fire resistant underlayment suitable for use in roofing systems and the like which can satisfy Class A and/or Class B UL-790 requirements.